Composites are often made to create a beneficial mix of the properties of dissimilar materials that are unobtainable in a single homogenous material. In the field of transducers for example, it can be advantageous to use a piezoelectric ceramic-polymer composite, rather than a monolithic tube of piezoelectric material such as lead zirconate titanate (PZT). Piezoelectric composites consist predominantly of a polarizable phase embedded in a non-polarizable material.
These composites have many advantages over traditional monolithic piezoelectric ceramics including: (i) lower densities resulting in acoustic impedance closer to those of the human body, water, etc., thereby eliminating the need for an acoustic matching layer; (ii) low dielectric constants resulting in a high piezoelectric voltage constant g; and (iii) ease of conformability to the shape of the backing material of the composite. Such composite piezoelectric transducers and methods for their production, are described, for example, in Composite Piezoelectric Transducer; R. E. Newnham et al.; Materials in Engineering, Vol. 2, December 1980, Pages 93-106, which is incorporated herein by reference.
Four composite designs that have been particularly successful are composites with 2-2, 3-2, 3-1, and 1-3 connectivity, in which piezoelectric ceramic sheets or network or rods are aligned in the poling direction of the composite and embedded in a matrix of a suitable polymer. In the case of the 2-2 composite, both the ceramic and polymer phases are two-dimensionally self-connected throughout the composite. The stiff ceramic phase supports most of the stress applied in the direction of its alignment, yielding a high piezoelectric charge coefficient d, while the composite has a low density and dielectric constant.
In the 3-2 composite, the ceramic phase is continuously self-connected in all three dimensions while the polymer phase is continuous only in two dimensions. The geometry and structure yields a high piezoelectric charge coefficient d and exhibits superior properties over single phase piezoelectric composites.
In the 1-3 composite, the ceramic phase is one-dimensionally self-connected through the composite, while the polymer phase is three-dimensionally self-connected. For some applications, the 1-3 composite yields superior properties to those described above for the 2-2 composite due to the lower density and dielectric constant.
In the 3-1 composite, the ceramic phase is three-dimensionally self-connected through the composite, while the polymer phase is one-dimensionally self-connected. This composite exhibits excellent electromechanical properties, and for some applications this structure may be preferred over the others described above.
Unlike other shaped composites which use the x, y, and z dimensions of the cartesian coordinate system to denote connectivity, cylindrically shaped composites have their connectivity denoted using cylindrical coordinates. Cylindrical coordinates are a generalization of two-dimensional polar coordinates to three-dimensions by superimposing a height and use the z, r, and .theta. dimensions. The r dimension defines the distance from the origin perpindicular to the z-axis. The .theta. dimension defines the extent to which an r dimension can be defined while revolving around the z-axis. The z-axis gives the height of the structure and is the same as in cartesian coordinates.
Recently, solid freeform fabrication techniques have been developed for producing three-dimensional articles without the need for molds, dies, or other tooling. One such technique, commercialized by Stratasys.TM., Inc. of Eden Prarie, Minn., builds solid objects layer by layer from polymer/wax compositions by using computer-aided design (CAD) software programs. According to the technique, which is described in U.S. Pat. No. 5,121,329 and is incorporated herein by reference, a flexible filament of the polymer/wax composition is fed by a pair of counter rotating rollers into a dispensing head which includes a liquifier and nozzle outlet. Inside the liquifier, the filament softens and melts at a temperature just above its melting point.
As the counter-rotating rollers continue to advance the solid filament into the liquifier, the force of the incoming solid filament extrudes the molten material out from the nozzle where it is deposited on a build platform positioned in close proximity to the dispensing head. The CAD software controls the movement of the dispensing head in the horizontal X-Y plane and controls the movement of the build platform in the Z direction. By controlling the processing variables, the extruded bead, called a "road", can be deposited layer by layer in areas defined from the CAD model, leading to the creation of an object that is a three-dimensional depiction of the CAD model.
Although the fused deposition technique is explained in detail above, other techniques, including, but not limited to, stereolithography, selective laser sintering, sanders prototyping, and laminated object manufacturing can be used in this invention. In stereolithography, for example, as described in U.S. Pat. No. 4,929,402, which is herein incorporated by reference, an ultraviolet curable polymer is used as a feed material and a computer controlled and focused beam of ultra violet rays is used to fabricate three dimensional objects.
In selective laser sintering, which is desribed in U.S. Pat. No. 4,938,816 and which is hereby incorporated by reference, a laser curable polymer is used as a feed material and a computer controlled and focused laser beam is used to fabricate three dimensional objects.
In Sanders.TM. Prototyping technology, which is described in U.S. Pat. No. 5,506,607 and is herein incorporated by reference, an ink jet printing process is used where a thermoplastic polymer is used instead of an ink. Build and support jets deposit thermoplastic polymer compositions based on a toolpath specified by the original CAD file. The build and support polymers are incompatible, allowing for the chemical dissolution of the support without damage to the build material. Three-dimensional objects are built by depositing layer upon layer of the thermoplastic polymers on a computer-controlled fixtureless platform. After the printed layer is complete, the platform indexes by one layer, and a milling bar passes over the layer to remove any excess material. After chemical dissolution of the support, the 3-D prototype made from the build material remains.
In Laminated Object Manufacturing (LOM), sheets of paper, polymer, or ceramic materials are deposited on top of each previous layer and a computer controlled laser beam is used to cut the sheet of material to make the three dimensional object.
In Fused Deposition of Materials which is described in U.S. Pat. No. 5,738,817 and which is hereby incorporated by reference, A 3-D modeling technology is employed to directly deposit green ceramic parts without the need for part-specific dies or tooling. In the case of FDM, a ceramic loaded filament is fed through a heated liquifier, which traverses a 2-D toolpath based on the slice of interest. After deposition is complete, the z-stage indexes by one layer, and the process repeats. After the build process is complete, the part is processed in a fashion similar to an injection-molded part.
This invention takes advantage of the aforementioned solid freeform fabrication technology which makes possible the efficient manufacture of such composites with phase geometries that have previously been impossible and/or impractical. The radial designs disclosed herein show considerably enhanced performance over currently available transducer designs.